Pattern of articular involvement and number of joints involved

The pattern of involvement and number of joints involved are important because the most likely diagnoses for monoarticular, oligoarticular and polyarticular arthritis vary. Management is dependent on the most likely diagnosis.

Monoarticular arthritis (monoarthritis) usually affects only 1 joint, although it may represent an evolving oligo- or polyarthritis. The most common causes are gout and calcium pyrophosphate deposition, but infection (eg septic arthritis) is very important to exclude.

Oligoarticular arthritis (oligoarthritis) affects 2 to 4 joints and is characteristic of spondyloarthritis. It may also represent an early or evolving polyarthritis. Septic arthritis can have an oligoarticular presentation in 10 to 20% of cases.

Polyarticular arthritis (polyarthritis) affects 5 or more joints and has many potential causes. A symmetrical polyarthritis is likely to be an inflammatory condition (eg rheumatoid arthritis in adults) and requires different management to a noninflammatory condition; see Inflammatory versus noninflammatory symptoms below.

Axial arthritis specifically involves the spine or sacroiliac joints but may involve other joints depending on the condition; see Assessment of back and neck pain and Spondyloarthritis for more detail.